Help! Dash cluster reporting incorrect average and instant MPG

[TedC]

At times the dash cluster is reporting 99 average MPG, with an instant MPG of 0. If I reset, it may correct itself. If I do nothing, after 10 mintes of driving, it may start to show correct values. Also noticed that when the average MPG shows high (99), so does the range (>600). At times, it will work properly for days. I've read the other posts on this topic, and tried resetting the ECM. A wild guess is that this is a sensor or ECM related issue. Any suggestions on how to pinpoint the cause would be appreciated.

 

[gmjunkie]

At times the dash cluster is reporting 99 average MPG, with an instant MPG of 0. If I reset, it may correct itself. If I do nothing, after 10 mintes of driving, it may start to show correct values. Also noticed that when the average MPG shows high (99), so does the range (>600). At times, it will work properly for days. I've read the other posts on this topic, and tried resetting the ECM. A wild guess is that this is a sensor or ECM related issue. Any suggestions on how to pinpoint the cause would be appreciated.

Check out the speed sensors on All 4 wheels and transmission,I've seen 1 of them not working properly cause this problem!!

 

[TedC]

Trip odomoter is working correctly all of the time. Resetting the MPG with the car stopped (park/drive) or with the ignition in the "on" position, will show 12.8 MPG then quickly resume to 99 average MPG. That reset is not working properly. I remember that my 1988 would flash the 12.8 on reset as well. ABS is working fine, as well as the MPH and RPM.

 

[TedC]

Decided to contact Gordon Killebrew on his fee consulting line (800-For Your VETTE). As I described the above situation, Gordon explained that there is a serial data issue with the stock ECMs, and was very confident that replacing the ECM would resolve it. When searching the internet, I found two other situations similar to mine (one was a 1986 and the other a 1989) that was fixed using a new ECM. I'll post an update when completed. Right now the weather is 60+ and sunny in Western New York. so I'm enjoying the ride.

 

[TedC]

As promised, here is the update. Gordon was correct and that the issue was the ECM. The ECM needs to send MPG data to the cluster via a serial data line. User access is via connector "E" on the ALDL port. Purchased a rebuild ECM from Advanced Auto for $80. It was Cardone Part # 777165 (AC Part # 1227165), which is used on the 1986-1989 vettes (other cars as well). I opened the case to verify Cardone did indeed clean and recoat the electronics with a special clear sealant when they rebuilt it. They claim improved ECM reliability over the GM sealant. Replacement was pretty easy. 3 screws hold the passanger kick panel, and 2 screws hold the ECM in place. Transferred PROM, ECM cover plate, and two mounting prongs.